/**
 * C++线程池，作者：https://github.com/TOMO-CAT
 * */

#ifndef THREAD_POOL_H
#define THREAD_POOL_H

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <utility>
#include <functional>
#include <stdexcept>

class ThreadPool {
public:
    explicit ThreadPool(size_t);
    template<typename F, typename... Args>
    auto enqueue(F&& f, Args&&... args)
        ->std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();

private:
    // need to keep track of threads so we can join them
    std::vector<std::thread> workers_;
    // the task queue
    std::queue<std::function<void()>> tasks_;

    // synchronization
    std::mutex queue_mutex_;
    std::condition_variable condition_;
    bool stop_;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads) : stop_(false) {
    for (size_t i = 0; i < threads; ++i) {
        workers_.emplace_back(
            [this] {
                for (;;) {
                    std::function<void()> task;
                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex_);
                        this->condition_.wait(lock,
                            [this] { return this->stop_ || !this->tasks_.empty(); });
                        if (this->stop_ && this->tasks_.empty()) {
                            return;
                        }
                        task = std::move(this->tasks_.front());
                        this->tasks_.pop();
                    }
                    task();
                }
            });
    }
}

// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args)
-> std::future<typename std::result_of<F(Args...)>::type> {
    using return_type = typename std::result_of<F(Args...)>::type;

    auto task = std::make_shared< std::packaged_task<return_type()>>(
        std::bind(std::forward<F>(f), std::forward<Args>(args)...));
    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex_);

        // don't allow enqueueing after stopping the pool
        if (stop_) {
            throw std::runtime_error("enqueue on stopped ThreadPool");
        }

        tasks_.emplace([task]() { (*task)(); });
    }
    condition_.notify_one();
    return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool() {
    {
        std::unique_lock<std::mutex> lock(queue_mutex_);
        stop_ = true;
    }
    condition_.notify_all();
    for (std::thread& worker : workers_) {
        worker.join();
    }
}

#endif  // THREAD_POOL_H